Well, I think you can't really compare them since cassettes area absolutely outdated.
Even if they would have a cassette mode in "animated" it simply wouldn't work anymore.
Today we have CDs or MP3s and the like. Personally I can't even remember the last time I've seen an audio cassette in a store.
So, for "animated" the guitar-modes are very well chosen as they still fit to Soundwave as a character.
But for G1 it was a very good idea to give Soundwave his cassette minions.
Back then it worked perfectly well.

So, for me both modes are equally good. I have no real preference here.

ARE Ratbat and Laserbeak more likeable as Cassettes or as Guitars, not is. Please do not basterdise the queens english! (im screwing around before anyone has a hissy)
Maybe not cassettes as they are out dated but I do prefer them being able to transform in to small things and store in Soundwaves chest rather than this giant robot walking around carrying two guitars around lol. Seems a little silly.
I always loved Soundwave activating a load of warriors ejecting from his chest, Like Barricade with Frenzy in the film.